Measuring transducer movement methods and systems for multi-dimensional ultrasound imaging

ABSTRACT

Methods and systems for measuring transducer movement are provided. For example, free-hand scanning for three-dimensional imaging is provided. An optical sensor within the transducer measures motion along the skin surface similar to measuring movement of a personal computer mouse with an optical sensor. Alternatively or additionally, the transducer is tilted at an angle other than perpendicular to the skin surface generally towards or away from the direction in which the transducer is translated. The transducer is then translated while maintaining the angle. Motion to or from the transducer is measured, and a component of the motion measured while the transducer is maintained at an angle is parallel to the direction of translation. The component of motion is angle corrected and used to determine a distance of travel.

RELATED APPLICATIONS

The present patent document is a divisional of Ser. No. 11/590,569, nowU.S. Pat. No. 7,597,664, filed Oct. 30, 2006, which is a continuation ofSer. No. 10/372,423, now U.S. Pat. No. 7,150,716, filed Feb. 20, 2003,which are both hereby incorporated by reference.

BACKGROUND

This invention relates to multi-dimensional ultrasound imaging. Inparticular, methods and systems for free-hand or user operated scanningfor three-dimensional (3D) ultrasound imaging are provided.

For 3D ultrasound imaging, a one-dimensional linear array transducer maybe used to acquire a plurality of two-dimensional frames of data orimages. A three-dimensional volume is scanned by obtaining a pluralityof two-dimensional images in different planes. For example, thetransducer is held in one location and rotated about an azimuthal axisto scan part of a cylindrical volume. Another example is translating thetransducer along an elevation dimension or perpendicular to the array ofelements over the surface of the skin. Knowing the relative positions ofeach scan plane to other scan planes results in more accuratethree-dimensional imaging. However, one-dimensional linear arraysprovide limited information of scan plane position relative to otherscan planes, leading to uncertainty and poor quality three-dimensionalrepresentations.

Various methods and devices have been developed for determining therelative position of scan planes. The decorrelation of ultrasound datafrom one scan plane relative to ultrasound data of an adjacent scanplane may indicate a distance between scan planes, such as disclosed inU.S. Pat. No. 5,876,342, the disclosure of which is incorporated hereinby reference. A mechanical structure to mount the transducer may be usedto accurately measure or position the transducer for scanning each ofspaced scan planes, but such brackets or mountings are difficult, timeconsuming and cumbersome. Magnetic position sensors or gyroscopes withinthe transducer provide signals indicating the movement and orientationof the transducer for determining the position of scan planes. Thesetypes of sensors may add undesirable weight and complexity to thetransducer and metallic objects in the examination area may adverselyaffect the performance of magnetic position sensors. Cameras, infraredsensors or other sensor positioned within a room for ultrasound scanningremotely identify the position of the transducer on the patient or LEDson the transducer on the patient, but obstructions may interfere withthese devices.

BRIEF SUMMARY

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on these claims. By way ofintroduction, the preferred embodiments described below include methodsand systems for measuring transducer movement in multi-dimensionalultrasound imaging. For example, free-hand scanning forthree-dimensional imaging is provided. An optical sensor within thetransducer measures motion along the skin surface similar to measuringmovement of a personal computer mouse with an optical sensor.Alternatively or additionally, the transducer is tilted at an angleother than perpendicular to the skin surface generally towards or awayfrom the direction in which the transducer is translated. The transduceris then translated while maintaining the angle. Motion to or from thetransducer is measured, and a component of the motion measured while thetransducer is maintained at an angle is parallel to the direction oftranslation. The component of motion is angle corrected and used todetermine a distance of travel.

In a first aspect, a system for measuring transducer movement formulti-dimensional ultrasound imaging is provided. An optical sensorconnects with the transducer. A processor is operable to determinemovement of the transducer in response to a signal from the opticalsensor.

In a second aspect, a method for measuring transducer movement formulti-dimensional ultrasound imaging is provided. The transducer ismoved. A distance of movement of the transducer is determined with anoptical sensor responsive to patient tissue.

In a third aspect, a method for measuring transducer movement formulti-dimensional ultrasound imaging is provided. A transducer ispositioned at an angle other than perpendicular to skin surface. Atleast a portion of the transducer is translated on the skin surface. Adistance of motion of the transducer during the translation is measuredfrom ultrasound data. The distance is a function of the angle.

In a fourth aspect, a system for measuring transducer movement formulti-dimensional ultrasound imaging is provided. The system includes atransducer. A processor is operable to determine a distance between scanplane positions as a function of a substantially maintained transducerangle other than perpendicular to a skin surface at each scan planeposition and as a function of measured motion from ultrasound data.

Further aspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram of one embodiment of an ultrasound system formulti-dimensional imaging.

FIGS. 2A and 2B are side and bottom views, respectively, of oneembodiment of an ultrasound transducer with an optical sensor;

FIG. 3 is flowchart diagram of one embodiment of a method for measuringtransducer movement for three-dimensional ultrasound imaging; and

FIG. 4 is a graphical representation according to one embodiment of atransducer translated along the skin surface at an angle.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Free-hand scanning for multi-dimensional imaging allows the operator totranslate or rotate the transducer. The movement of the transducerallows scanning of multiple two-dimensional planes within a volume. Toprovide geometric accuracy for the relative positioning of the varioustwo-dimensional planes, an optical sensor is positioned on thetransducer or motion data is measured while scanning with the transducerat a non-perpendicular angle to the skin surface. Either technique fordetermining the position of scan planes is used independently or incombination. When used in combination, the scan plane positions areaveraged, combined or used as verification. Each of these two approacheswill be discussed below with some description, such as three-dimensionalrendering and the ultrasound system, being either the same for both.

FIG. 1 shows one embodiment of an ultrasound system 10 for measuringtransducer movement for multi-dimensional ultrasound imaging. The system10 includes a transducer 12, a beamformer 14, a detector 16, a motionprocessor 18, a 3D processor 20, a display 22 and a user input 24.Additional, different or fewer components may be provided, such as usinga control processor to implement both the motion processor 18 and the 3Dprocessor 20.

The transducer 12 comprises a linear or multi-dimensional array ofelements, such as PZT elements or CMUT elements. The array of elementsis configured as a linear, curved linear or other array for linear,sector, or Vector® imaging. In one embodiment, the transducer 12 isadapted for use on the exterior of a patient or used in a patient's skinsurface. For example, the transducer 12 is enclosed within a housingadapted to be held by a user's hand. The transducer 12 is positionedadjacent to the housing surface of the probe such that the transducer isoperable to transmit acoustic energy through the housing surface, suchas through a lens or acoustic window. The housing surface is generallyplanar or slightly curved, smooth surface for contacting the pliableskin surface. In other embodiments, the transducer 12 is adapted forinsertion within the patient, such as a catheter, transesophageal orendocavity probe. Any of various now known or later developedtransducers 12 may be used.

In one embodiment shown in FIGS. 2A and 2B, the transducer 12 includesan optical sensor 28. The optical sensor 28 comprises a light source 30and a camera 32. In alternative embodiments, the light source 30 is notprovided or is separate from the transducer 12. The light source 30comprises a light emitting diode. The light source 30 emits light at anyof various wavelengths in the optical spectrum. In one embodiment, thelight source 30 emits light at a wavelength of less than 700 nanometers,such as a blue green LED emitting a wavelength of around 400 nanometers.A red LED emitting a wavelength around 700 nanometers may be used.Shorter wavelengths are more likely to reflect from the skin surfacerather than pass through tissue. Alternatively, the patient tissue iscoated with ink or another substance for darkening the skin and makingit more reflective of a wavelength of light detected by the opticalsensor 28.

The camera 32 comprises a complementary metal-oxide semiconductor (CMOS)sensor. In alternative embodiments, a CCD, photodetector cells or otheroptical wavelength sensor is provided. The light source 30 bounces lightoff of the skin surface or other tissue onto the camera 32. The camera32 detects or senses images, such as hundreds or thousands of imagesevery second, at least in part of the wavelength of the light source 30.In one embodiment, the optical sensor 28 comprises the structures and/ormethods disclosed in U.S. Pat. Nos. 4,794,384; 6,330,057; and 6,246,482,the disclosures of which are incorporated herein by reference. Any ofspeckle, phase shifts, phasing, frequency, or other characteristics ofreceived light may be used to detect motion.

The optical sensor 28 is positioned on, within or adjacent to thehousing surface of the transducer 12. The optical sensor 28 receiveslight directed towards the housing surface. By positioning the opticalsensor 28 adjacent to an acoustic window 34, the linear array behind theacoustic window 34 or on an outer surface adapted for placement adjacentto or in contact with the tissue surface, the optical sensor 28 ispositioned to receive light from the skin surface. In alternativeembodiments, the optical sensor 28 is positioned away from the skinsurface but directed towards the skin surface for measuring movement ofthe transducer 12. In yet other alternative embodiments, multipleoptical sensors 28 are provided on the transducer 12.

In an alternative embodiment, an adaptor with the optical sensor 28connects with the transducer and the housing. The adaptor snap fits,latches, screws or otherwise connects with the transducer 12. Theadaptor is provided with an acoustic window in one embodiment forconnecting between the transducer 12 and the skin surface or tissuesurface during use, but in alternative embodiments connects to a side ofthe transducer 12 for positioning the optical sensor 28 adjacent to theskin surface and adjacent to the surface of the transducer 12 adaptedfor contact with the skin. In yet other alternative embodiments, theadaptor connects elsewhere on the transducer 12.

The beamformer 14 comprises any of various analog and/or digitalcircuits for receive and transmit beamforming. The beamformer 14generates transmit waveforms for the transducer 12, and beamformsultrasound data from received echo signals in response to various delaysand apodization. The detector 16 comprises a B-mode detector, a flowdetector, a Doppler detector, a harmonic detector, a contrast agentdetector or other now known or later developed detector of informationfrom the beamformed ultrasound data.

The motion processor 18 comprises a general processor, a digital signalprocessor, an application specific integrated circuit, an analogcircuit, a digital circuit, combinations thereof, or any other circuitsnow known or later developed for determining motion from the ultrasounddata or from the optical sensor 28. In the embodiment using the opticalsensor 28, the motion processor 18 is housed within the transducer 12,in an adaptor connected with the transducer 12 or with other componentsof the ultrasound system 10, such as within the data processing path asshown or not within the data processing path. For use with an opticalsensor 28, the motion processor 18 comprises a digital signal processoroperating at millions of instructions per second (e.g., 18 MIPS) todetermine a translation along at least one axis. For example, the motionprocessor 18 determines the direction on a two-dimensional surface anddistance along the direction corresponding to translation of thetransducer 12 based on pattern matches. The optical sensor 28 inconjunction with the motion processor 18 may also be operable todetermine rotation based on the pattern matching or correlation process.

In the embodiment of detecting motion from ultrasound data acquired withthe transducer 12 held at an angle, the motion processor 18 comprises,in part, a flow detector 16 operable to detect motion along a scan lineand a same or additional processor operable to determine a component ofmotion parallel to a skin surface as a function of the motion detectedalong the scan line. Any combination of single processors or multipleprocessors provided in one component or spaced throughout the ultrasoundsystem 10 or elsewhere may be used. The motion processor 18 is operableto determine a distance between scan plane positions as a function of asubstantially maintained transducer angle other than perpendicular tothe skin surface at each scan plane position and as a function ofmeasured motion of translation of the transducer 12 between the scanplane positions. Using a determined velocity, such a Doppler velocity ora cross correlation, the motion processor 18 is operable to measure inplane motion and extract a distance between scan planes from the motion.The motion processor 18 in one embodiment is also operable to render athree-dimensional representation as a function of the distance, but aseparate processor may be provided as shown in FIG. 1. In oneembodiment, the motion processor 18 operates on detected ultrasounddata. In an alternative embodiment, the motion processor 18 receivesbeamformed ultrasound data prior to detection for determining crosscorrelation or other indicators of motion.

The user input 24 comprises a keyboard, dedicated keys, softwareprogrammable keys, touch screen, knobs, switches, sliders, joystick,trackball, mouse, combinations thereof or any now known or laterdeveloped user input devices. In the embodiment for detecting transducermotion based on in scan plane detected motion and the angle of thetransducer, the user in input 24 connects with the motion processor 18.The user inputs the angle for holding the transducer. In alternativeembodiments, a tilt sensor within the transducer 12 automaticallydetermines the angle, or a fixture is used to position the transducer 12at a desired angle. In yet other alternative embodiments, the user isinstructed to hold the transducer 12 at a particular angle rather thaninputting an angle. For either of the optical sensor or angledtransducer embodiments, the user input 24 may be used for calibratingthe system 10 or motion processor 18.

The three-dimensional processor 20 comprises a processor, a digitalsignal processor, an application specific integrated circuit, a videocard, graphics accelerator, digital circuit, analog circuit,combinations thereof or any other now known or later developed devicefor rendering a representation of a three-dimensional volume. Atwo-dimensional image representing a 3D volume is rendered fromultrasound data associated with a plurality of scan planes. The scanplanes are spaced within a volume such that the rendering is responsiveto the relative positions of the scan planes. The relative positioningof the scan planes is determined as a function of the movement of thetransducer 12 or the distance detected by the motion processor 18. Inone embodiment, the 3D processor 20 is configurable to perform differenttypes of volume rendering, shading, surface rendering or texturemapping. In other alternative embodiments, the 3D processor 20 comprisesa dedicated circuit for performing a particular type of rendering usingdata interpolated from a Cartesian coordinate, polar, cylindrical orother specific or known grid. The distance or motion information is usedto interpolate the data onto the grid.

FIG. 3 shows a flow diagram of one embodiment of a method for measuringtransducer movement for three-dimensional ultrasound imaging. The methoduses one or both of measuring transducer movement with an optical sensor28 or measuring transducer movement as a function of a measured motionand angled position of the transducer 12. Other methods with additional,different, or fewer acts may be used.

In act 50, the transducer 12 is positioned for imaging and moved. Forexample, and as shown in FIG. 4, the transducer 12 is positioned alongthe surface of the skin 60 in a first position A and moved to a secondposition B, a change represented by ΔAB. In one embodiment, thetransducer 12 is positioned perpendicular to the skin surface 60. Inanother embodiment, the transducer 12 is positioned at an angle otherthan perpendicular to the skin surface 60 or other tissue. For example,the transducer 12 is rotated away from a normal to the skin surface byan angle α as shown in FIG. 4, defining a triangle having sides ΔAB, Δr,and an orthogonal line between the scan planes 62. α can be of any ofvarious angles, such as about 60 degrees. About and substantially areused herein for describing an angle or transducer position to accountfor user estimation, user changes in transducer position away fromintended motion due to the inexactness of user control or free-handmovement along the often bumpy or flexible skin surface, patientbreathing, patient movement or other variable. The transducer 12 is at anon-perpendicular angle in the out-of-plane dimension in one embodiment.Where the transducer 12 is positioned at an angle to the normal as shownin FIG. 4, a linear array of the transducer 12 is positioned such thatthe both ends of the array are about a same distance from a skinsurface, such as associated with rotating the transducer 12 away fromthe normal around the azimuthal axis. As a result, the scan planes 62are seen edge on in FIG. 4 (e.g. the scan planes extend perpendicular tothe plane of the page or paper). Alternatively, one end of an array isrotated further away from the skin surface than the other end.

For the embodiment using the angled transducer 12 as shown in FIG. 4, atleast a portion of the transducer 12 is translated on the skin surface60. For example, the transducer 12 is translated along an elevationdimension from the position A to the position B. The angle α issubstantially maintained during translation or at least at each locationwhere a scan plane 62 is acquired. As another example, the transducer 12is rotated about a center axis extending along the range dimension whilemaintaining the range dimension axis at an angle away from the normal tothe skin surface 60 or other tissue surface. Any one or combinations ofrotational, linear and two-dimensional translation may be used.

In act 52, the distance, motion, rotation, direction or othercharacteristic of transducer movement is determined. One or both of thetwo different acts 54 and 56 may be used for determining the motion orother characteristic. In act 56 the motion of the transducer 12 ismeasured from ultrasound data. The motion is determined as a function ofthe angle of the transducer 12 away from perpendicular. The ultrasounddata is used to determine motion along or within the scan plane 62, suchas a long one or more scan lines. For example, a Doppler velocity isdetected along one or more depths within a scan line or the imagingplane 62. As another example, a cross correlation of beamformed dataprior to detection or after detection is performed. The crosscorrelation indicates the velocity or motion relative to the transducer12. Other techniques for determining motion to or away from thetransducer 12 may be used. For example, the peak of the crosscorrelation function between two data sets indicates the velocity alongthe scan line or image plane 62. The sign of the shift indicates thedirection of the motion, indicating the direction of transducer motion.

The component of motion parallel to the skin surface 60 is extractedfrom the measured motion. For example, the measured motion is divided bythe sine of the transducer angle α. An average of multiple measurements,a single measurement, different measurement for different transducerangles or other combinations of measurements may be used. Since the timeof acquisition between the two different scan planes 62 is known, thecomponent of motion along the skin surface 60 is converted to a distancebetween the two scan planes 62. Given the angle and the distance, therelative position of each scan plane 62 to other scan planes is known.

At each transducer position or a range of positions during continuousmovement, a two-dimensional scan is performed. The two-dimensional imagedata may include tissue motion compensation to counteract the motion ofthe transducer 12 during the two-dimensional scan. For example, a motionrelative to different regions within a scan plane is determined. Theestimated position of the scan plane is warped or adjusted as a functionof the different positions of various scan lines within the imagerelative to the ideal scan plane position 62. Flow estimation techniqueslike Doppler, color flow or spectral Doppler measure the motion, ormotion is estimated in the time domain by cross correlation or sum ofabsolute differences calculations. Other calculations or measurements ofmotion from ultrasound data may be used.

In act 54, the distance or other motion characteristic is measured withthe optical sensor 28. The optical sensor 28 is responsive to patienttissue, such as detecting light reflected from or through skin tissueusing correlation, phase shift or other techniques for determining adirection and amount of translation of the transducer 12. Using theoptical sensor 28, the transducer 12 is positioned perpendicular ornon-perpendicular to the tissue surface 60. By directing the opticalsensor 28 towards the skin surface, the distance of transducer movementis determined based on the changes in position of the transducer 12 overthe skin surface. By positioning the optical sensor 28 adjacent to thetransducer 12 or as part of the transducer 12, the distance isdetermined by a camera facing in the same direction as the transducer 12or passing over similar skin surfaces. The optical sensor 28 may alsodetect rotational movement along a single axis or movement alongmultiple axes in a plane.

Since the transducer 12 may block light from the skin surface adjacentto the camera or optical sensor 28, the optical sensor includes an LEDor other light source 30. In one embodiment, light is directed towardsthe patient tissue and reflected back to the camera 30 of the opticalsensor 28. In alternative embodiments, the optical sensor 28 ispositioned to receive light reflected from tissue not covered by thetransducer 12.

The optical sensor 28 is calibrated by the user or by the manufacturer.In alternative embodiments, empirical testing or other measurementsallow programming the optical sensor 28 in the motion processor 18without further calibration. For calibration, the image characteristicsused by the optical sensor 28 are adjusted for known distances across apiece of paper or sample tissue. Calibration may minimize errors in thedistance calculation from the optical sensor 28.

In act 58, a three-dimensional representation image is rendered. Thethree-dimensional representation is rendered as a function of thedistance or other motion characteristic. The ultrasound data, such asdetected velocity, intensity, energy, power or other data is organizedas image data frames for each scan plane position. The ultrasound datacomprises scan converted data in a Cartesian coordinate system or polarcoordinate data. The image data frames are associated with relativepositional information, such as the distance, angle, relative rotation,orientation and/or other positional information. The two-dimensionalimage frames are either coplanar or non-coplanar, such as two or morerotationally offset planes or two or more planes offset in an elevationposition. The positional information provides the relative positionamong the image data frames so that these frames may be subsequentlyassembled in a three-dimensional volume, such as along a Cartesiancoordinate or a polar coordinate three-dimensional grid. In oneembodiment, the positional information comprises three components ofposition (X, Y, Z) and three components of rotation (about X, about Yand about Z). Other definitions of position and orientation may be used,such as two known points and one origin point on each plane.

The information from the two-dimensional image data frames is convertedto the 3D grid, such as regularly (equal) spaced volume grid. Equalspacing allows for efficient calculations and use with low costvisualization software, but unequal spacing may be used. The conversionis performed by interpolation, nearest neighbor selection or other two-or three-dimensional conversion techniques. Alternatively, therepresentation is rendered from data not converted to a specific 3Dgrid.

Appropriate data is selected from the 3D grid samples or from data noton the regular 3D grid to provide a desired representation. Any ofvarious three-dimensional imaging techniques may be used, such asharmonic or fundamental data three-dimensional imaging disclosed in U.S.Pat. No. 5,928,151, the disclosure of which is incorporated herein byreference. The selected information is used for surface rendering,volume rendering or other three-dimensional rendering technique. Forsurface rendering, thresholding or other processes identify one or moresurfaces. Once the surfaces are determined, a polygon mesh is formed torepresent the surface. The surface is rendered with lighting cues, suchas Gouraud or Phong shading.

Any of alpha blending, maximum intensity projection or minimum intensityprojection may be used for volume rendering. In response to a userselected or system determined viewing angle, data along ray linesparallel with the viewing angle intersect the 3D data grid. The dataalong each line is averaged or selected for a minimum or maximumintensity value. By alpha blending or selecting maximum or minimumvalues along each of the ray lines, a two-dimensional representation ofthe three-dimensional volume is generated as seen from a particularviewing angle. Other representations associated with a different viewingangles may be generated for rotating the representation for a morethree-dimensional appearance.

Either or both of the optical sensor 28 or angled transducer 12 withmotion measurement may be used for extended field of view or panoramicimaging. A plurality of coplanar images associated with differenttransducer positions are acquired. The distance between transducerpositions is used to align the images for compositing as an extendedfield of view.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. Forexample, any of various perpendicular or angled positions of transducers12 relative to the skin surface 60 or orientation of the transducerarray may be used, including combinations of different angles. Amulti-dimensional array, such as 1.25, 1.5, 1.75, 2D arrays may be used.Sensors or brackets in additional to an optical sensor may be used.Transducers 12 without an optical sensor may also be used. Any ofvarious now known or later developed optical sensors using patternmatching or other characteristics for determining a distance may beused. The distance may be a relative spacing without an actual spacingdetermination.

It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, that areintended to define the spirit and scope of this invention.

We claim:
 1. A method for measuring transducer movement formulti-dimensional ultrasound imaging, the method comprising: moving, byfree-hand motion, a transducer across a patient's skin; positioning thetransducer at an angle other than perpendicular to the patient's skin;scanning a plurality of planes below the patients skin with ultrasoundfrom the transducer, the plurality of planes at different locationswithin the patient due to the moving; translating at least a portion ofthe transducer on the patient's skin during the scanning whereintranslating comprises translating the transducer while substantiallymaintaining the angle; measuring, from ultrasound data, a first distanceof the transducer translation during translating, the first distancebeing a function of the angle; directing an optical sensor and ablue-green light source at the patients skin during the moving;determining the different locations of the plurality of planes from thefirst distance and with the optical sensor responsive to blue-greenlight from the blue-green light source reflected from the patient skin;and rendering, from ultrasound data responsive to the scanning of theplurality of planes, a three-dimensional representation, the renderingusing positions of the planes, represented by the ultrasound data, thatare a function of the different locations.
 2. The method of claim 1wherein directing blue-green light comprises directing light with awavelength around 400 nanometers.
 3. The method of claim 1 whereindetermining comprises averaging the first distance with a seconddistance determined with the optical sensor.
 4. The method of claim 1further comprising: determining a direction of translation movement ofthe transducer, a rotational movement, or combinations thereof.
 5. Asystem for measuring transducer movement for multi-dimensionalultrasound imaging, the system comprising: a transducer positionedadjacent a housing surface of a probe such that the transducer isoperable to transmit acoustic energy through the housing surface andinto a patient; an optical sensor connected with the transducer, theoptical sensor comprising a blue-green light source and a camera, thecamera directed to receive light reflected of patient skin from theblue-green light; and a processor operable to determine, in response toa signal from the optical sensor, relative locations of scan planes dueto movement of the transducer wherein the processor is operable todetermine the relative locations as a function of a substantiallymaintained transducer angle other than perpendicular to the patient skinand as a function of measured motion from ultrasound data.
 6. The systemof claim 5 wherein the camera comprises a complementary metal-oxidesemiconductor sensor.
 7. The system of claim 5 further comprising athree-dimensional imaging processor operable to render an image fromultrasound data associated with the scan planes, the renderingresponsive to the relative locations of the scan planes.
 8. The methodof claim 5 wherein positioning comprises positioning the transducer atabout 60 degrees from perpendicular to the patient's skin, thetransducer comprising a linear array with both ends of the array about asame distance from the skin surface.
 9. The method of claim 5 whereinmeasuring comprises measuring a first motion along a scan line anddetermining a second motion parallel to the patient's skin as a functionof the angle and the first motion.
 10. The method of claim 9 wherein thefirst motion comprises a Doppler velocity detected from ultrasound dataalong the scan line.
 11. The method of claim 9 wherein measuringcomprises measuring the first motion in response to a cross-correlationof beamformed data prior to detection.
 12. The system of claim 5 furthercomprising a user input connected with the processor, the user inputoperable to receive an indication of the angle.
 13. The system of claim5 wherein the processor comprises, in part, a flow detector operable todetect a first motion along a scan line, the processor operable todetermine a second motion parallel to the skin surface as a function ofthe angle and the first motion.
 14. The system of claim 5 wherein theprocessor is operable to measure the motion in response to across-correlation of beamformed data prior to detection.
 15. The systemof claim 5 further comprising: an adaptor connected with the transducer,wherein the optical sensor is on the adaptor such that the opticalsensor is directed towards the patient skin.